The discovery of carbon nanotubes triggered a worldwide research effort devoted to determining their structure, calculating and measuring their physical properties, and to improving methods of production. Carbon nanotubes have many extraordinary physical and chemical properties, which has prompted their widespread use in many applications.
For example, they can be used as supports for metal catalysts.[1] As tubular structures, they have unusual capillary properties.[2] Mechanically, nanotubes are significantly stiffer than currently commercially available carbon fibers, and can therefore be used to strengthen composite materials or atomic force microscope tips.[3] Filled with metals or semiconductors, nanotubes may well provide components for nanoscale electrical or electronic devices such as amplifiers, switches or electrical-mechanical converters.
Three technologies have been used for the synthesis of carbon nanotubes: carbon-arc discharge, laser-ablation and catalytic decomposition processes.
In the carbon arc-discharge method, carbon nanotubes are grown in an inert gas atmosphere between carbon electrodes between which an electric arc is generated. The anode electrode is consumed to form a plasma, the temperature of which can reach 6000° C.[4]
The laser-ablation technique consists in exposing a graphite target to a pulsed or continuous high-energy laser beam. The graphite is either vaporized or fragmented into aggregates of a few atoms.[5]
The carbon-arc discharge and laser-ablation methods require highly technical equipment, and are quite expensive. As such, they were designed primarily for carbon nanotubes synthesis on a laboratory scale and were used primarily for theoretical investigation. They are not suitable for the large-scale production of carbon nanotubes.
The third technology that was implemented to make carbon nanotubes is catalytic decomposition of hydrocarbons or oxygen containing compounds in the presence of supported transition metal catalysts. This method is more amenable to industrial scale applications. However, current methods suffer the disadvantage that they use non-renewable fuels such as methane or hydrocarbons. The most widely studied method is catalytic decomposition of methane primarily on iron oxide,[6] but also on Ni/SiO2.[7]
Hydrogen gas is also a material of great industrial interest. It is being used primarily in processes for the desulphurization and/or hydrogenation of aromatic derivatives produced in oil refinery plants. It is also used in mixtures with carbon monoxide and nitrogen gas in the synthesis of methanol, ammonia and liquid hydrocarbon products (Fischer-Tropsch reactions).
The utilization of hydrogen gas for fuel-cell type applications is considered to be one of the most promising leads to answer the energy needs of the future[8]
So far, 96% of the hydrogen gas that is produced today originates from reforming of natural gas, primarily methane, (76%) and of light naphtha (20%). 4% of the production of hydrogen gas comes from partial oxidation of oil or petroleum residues.[9]
The reforming reaction is generally carried out at high temperature (400-700° C.) in the presence of an alumina supported Nickel catalyst. Generally, the reforming reaction is followed by a reaction in the presence of water (gas-to-water reaction) so as to oxidize carbon monoxide to carbon dioxide.
However, the main drawback associated with the production of hydrogen gas through reforming methane is that the process produces carbon dioxide. Likewise, current technologies for the production of carbon nanotubes utilize non-renewable fuels, such as natural gas or hydrocarbons. Therefore, the primary processes for producing hydrogen gas and carbon nanotubes currently used are not environmentally friendly.
Therefore, there remains a need to develop new processes for producing hydrogen gas and carbon nanotubes from renewable fuels, and without emission of pollutants that are harmful to the environment.